1. Field of the Invention
The present invention relates to an image-forming system which is constructed so that an optical beam such as a laser beam is exposed and scanned on an image-forming medium, such as a printer, a copying machine or a facsimile. More particularly, the present invention relates to a control apparatus for this image-forming system, which detects dislocation of the focal point of an optical beam and corrects the detected dislocation of the focal point automatically.
2. Description of the Related Art
Recently, an image-forming system comprising a laser writing unit where a laser beam modulated according to an image signal is exposed and scanned on a photosensitive image-forming medium by a polygonal mirror scanner, such as a printer, a copying machine or a facsimile, is widely and ordinarily used.
It is desired that an image information having a high quality and a high density will be obtained by this image-forming system. However, it often happens that dislocation of the focal point of the laser beam is caused by a change of the surrounding environment, and the beam diameter on the image-forming medium is caused by this dislocation of the focal point, resulting in reduction of the image quality.
As the means for overcoming this disadvantage, there can be mentioned a method as disclosed in Japanese Unexamined Patent Publication No. 60-100113 or Japanese Unexamined Patent Publication No. 02-51119. According to this known method, there are disposed detecting means for receiving a laser beam to be scanned on an image-forming medium and detecting dislocation of the focal point of the laser beam, and means for correcting the detected dislocation of the focal point of the laser beam by movement of a lens.
However, the laser printer disclosed in Japanese Unexamined Patent Publication No. 60-100113 is defective in that since a 4-divided light-receiving element is used as the means for detecting dislocation of the focal point and dislocation of the focal point is detected by the astigmatism, it is difficult to stably detect dislocation of the focal point of the laser beam, which is scanned at a high speed.
The scanning optical apparatus disclosed in Japanese Unexamined Patent Publication No. 02-51119 is constructed so that detecting means for detecting dislocation of the focal point of a laser beam is located at a position separate from an image-forming medium and optically equivalent to the image-forming medium to detect the bundle-state of a laser beam formed by the terminal portion of a group of lenses such as f.theta. lens. Accordingly, dislocation of the focal point cannot be accurately detected unless the lens characteristics of the terminal portion of the lens group are especially improved.